Soap Bubbles
and Voids

As noted in the preceding section, the distribution of luminous matter in the Universe has the character of soap bubbles and filaments. The following figure illustrates.

FIGURE: Data from the survey of galaxies. The voids and "walls" that form the large-scale structure are mapped here by 11,000 galaxies. Our galaxy, the Milky Way, is at the center. The outer radius is at a distance of approximately 450 million light-years. Obscuration by the plane of the Milky Way is responsible for the missing pie-shaped sectors to the right and left. Click on the image to get a larger version. (Smithsonian Astrophysical Observatory, 1993. Northern data (top)--Margaret Geller and John Huchra, Southern data (bottom)--Luiz da Costa et al. Quoted in Cosmology, a Research Briefing, National Academy of Sciences.)

Computer Simulation of Structure

Here is a link to a set of MPEG movies showing computer simulations of large-scale structural growth in the Universe that uses a mixture of hot and cold dark matter. The adjacent image shows the end result of such a simulation. In this simulation (Greg L. Bryan and Michael L. Norman, Grand Challenge Cosmology Consortium), filaments grow that eventually exceed 100,000 light years in length and surround gigantic bubbles of low-density gas. The intersections of the filaments produce regions of high density (signified by red) that are thought to generate bright X-ray clusters of galaxies.

The adjacent image shows a simulation of the growth of structure in the Universe using cold dark matter. The void and filament structure is quite noticeable in this image (Source).

Ahead, Warp Factor One, Mr. Sulu!

Here is a corresponding VRML 3D simulation of filament and soap bubble structure for the Universe using cold dark matter (Source). [Note: file is 2.1 MB in length and requires a VRML viewer (e.g., Cosmo Player).] If you have a VRML viewer, this simulation of a 100 Mpc x 100 Mpc x 100 Mpc cube of the Universe permits you to navigate through the filament, soap bubble, and void structure described above. The large open areas are the voids. The superclusters of galaxies are the dense clusterings along the filaments, and the brightest spots in these filaments are the most luminous large clusters.

How Were the Voids Formed?

Any theory of the origin of large scale structure must account for the formation of voids, which occur on scales of approximately 100 Mpc. We can immediately place one constraint on their formation. If we take as a reasonable estimate of a peculiar velocity for a galaxy ~ 600 km/s, it would take 160 billion years for such a galaxy to travel 100 Mpc and cross a void - far longer than the age of the Universe. Therefore, it is extremely unlikely that the voids were formed by galaxies moving out of a region the size of a void after their formation. The galaxies must (on the scale set by voids) have formed near where they are today, so voids must reflect the distribution of our present galaxies at the time when they were created.

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